Zenith AM-FM G725 - A825

[DavGoodlin]

Originally Posted by Electronic M

Just a dumb thought: have you tried subbing the osc tubes in case there's an internal tube defect skewing frequency?

Good idea though Im used to not having it easy with "just a tube", I swapped a 12DT8 and several 12AT7, no real difference. I even took unused pin 9 out of 12AT7 socket, got one MC closer from that. No ears are on socket for shield, adding one actually made things worse, with image rejection. Now down to 98.5 with slug bottomed out using a Tung-Sol 12AT7, best of show.

"Padding" the osc by subbing in values just seemed to make it hotter, with one or two strong FM stations taking over the range.

The string-spring loaded-slug German 12DT8 tuners used in RCA and Motorola were similarly hard to tune down. In theory, increasing capacitance and/or inductance lowers the resonant frequency.

I suspect the oscillator coil has deformed, I honed out the forms so slug goes all the way in, lots of action when I unwind some of it. I have a great selection of fractional pf caps, disc up to 470pf. "Padding" the osc by subbing in values just seemed to make it hotter, with one or two strong FM stations taking over the range.